This invention relates to the detection of geologic features and, more particularly, to the detection of marine gas seeps.
Natural gas seeps are widespread underneath the world""s oceans and seas. Gas, predominantly methane, is generated by the bacterial decomposition of organic matter in shallow sediments. Thermal cracking of hydrocarbons at greater depths also generates gas. Where the seafloor depth is greater than about 500 meters below sea level, and where temperatures are sufficiently low, this gas will combine with water to form gas hydrate. Gas hydrate is a form of ice in which a considerable amount of natural gas is trapped in the crystallographic cages formed by solid water. Hydrate is typically found in a band a few hundred meters thick below the seafloor.
The petroleum industry is very interested in detecting the presence of gas hydrate. There are formidable technical problems connected with drilling wells on the continental shelf and continental slope in the presence of hydrates. Solid hydrate frequently acts to cement sediments in which it exists, and this semi-consolidated mass may overlay highly fluidized unconsolidated sediment residing below the lower boundary of the hydrate stability zone. This is very similar to the conditions that cause avalanches on snow-covered slopes. In fact there is evidence of massive subsea slumps in areas known to have significant accumulations of hydrates. Such slumps can be significant hazards to oil and gas exploration and production operations.
Gas hydrates may also become a significant source of fossil fuel in the future. Enormous quantities of natural gas are trapped in hydrate reservoirs just beneath the seafloor. Many of these deposits are found in the exclusive economic zones of the United States, Canada, Japan, and Russia, relatively near energy consumers. While seismic prospecting has located many such deposits, it has been found that this method has missed some sizable accumulations.
Geochemical exploration surveys have been used to map the presence and distribution of oil and gas seepage and to help identify areas with a high potential for petroleum reservoirs. Exploration Technologies, Inc. of Houston, Tex., for instance, advertises that they have developed a wide range of marine geochemical sampling tools, including sediment coring, geochemical drilling, bottom water sampling, surface slick sampling, and a Sniffer system. The Sniffer system reportedly pumps a continuous stream of sea water from a height of approximately 10 meters above the seabed to one or more gas chromatographs located aboard a ship that continuously analyzes xe2x80x9cstripped gases for methane through butane light hydrocarbonsxe2x80x9d. Disadvantages of the Sniffer system apparently include the following: it is limited to water depths of about 600 feet or less; it may be difficult to deploy on seismic vessels with limited space; it is generally limited to light hydrocarbon analysis; it may not give reliable results in areas with very low seepage rates; and it has limited availability. Any analysis method that transports samples from near the seafloor to the water""s surface introduces potential sample handling problems and increases the delay time between sample collection and analysis. An improved method and apparatus for locating marine gas seeps is clearly desirable.
It is not uncommon for deep-sea equipment to observe bubbles of methane rising from the seafloor. It has also been observed that where no rising bubbles are apparent, disturbing the sediment will sometimes release gas. In still other instances, a sample of seawater will outgas when transported upwards in the water column. These observations indicate that methane is at or near its bubble point in seawater at many locations.
Even if methane is dissolved in water at its saturation concentration, bubble production may not occur. The gas phase can be thermodynamically stable at a given temperature and pressure, but a gas bubble cannot form because its surface free energy exceeds the free energy difference of the bulk phases. This phenomenon accounts for supercooling, superheating, or supersaturation commonly observed at first order phase transitions, and is described by classical nucleation theory. See, for instance, A. W. Adamson, xe2x80x9cPhysical Chemistry of Surfacesxe2x80x9d, 3rd edition, Wiley, 1976, chap. 8.
Other types of gases may be released by marine gas seeps, including carbon dioxide, nitrogen, and hydrogen sulfide. One potential application for an improved marine gas seep detection system is to monitor subsea reservoirs in which carbon dioxide is being sequestered or natural gas is being stored.
For these reasons, it would be of great benefit to be able to identify gas seeps with a local probe that can be deployed on or near the seafloor.
One aspect of the invention involves a method of detecting a marine gas seep that includes: deploying a local probe on or near the seafloor; producing bubbles in water near or within the local probe; detecting the bubbles; producing data indicating the relative concentration of dissolved gas in the water; and associating elevated dissolved gas concentrations with the presence of a nearby marine gas seep. Another aspect of the invention involves an apparatus configured to carry out the inventive method. Preferred embodiments of the invention utilize an ultrasonic transducer to both produce bubbles and detect them. Further details and features of the invention will become more readily apparent from the detailed description that follows.